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SWIR Imaging in Hot Process Monitoring
by Martin H. Ettenberg and Douglas S. Malchow - UTC Aerospace Systems (Sensors Unlimited Products) Posted 10/23/2006
The shortwave infrared (SWIR) band is now used in industrial process monitoring even in thermography applications, typically those of 100° C or more. It is extremely valuable for inspection of hot glass processes because SWIR cameras, based on indium gallium arsenide (InGaAs) technology, can image through glass, allowing the user to inspect for defects inside hot glass bottles or containers. Simultaneously, InGaAs-SWIR imaging can monitor the temperature uniformity during the cool down process. Typically, long wave infrared (LWIR) or mid-wave infrared (MWIR) thermal cameras utilizing microbolometer. indium antimonide (InSb) or mercury cadmium telluride (HgCdTe) detectors, cannot see inside the glass container being inspected. Thus the imagery provided by these thermal cameras is only a temperature measurement on the outside surface of the hot glass container.
SWIR cameras can easily image through the standard safety glass or quartz window enclosures that are used to protect cameras and other equipment in harsh industrial environments. This allows InGaAs SWIR cameras to inspect furnaces and hot metal processes more easily than other typical technologies which use longer wavelength light. The use of long wavelength imagers often requires exotic lenses or expensive window materials. These materials can be cost prohibitive in monitoring hot processes, especially when the enclosures are subjected to hot liquid splatter which often requires periodic replacement of the protective window. (see Fig. 1- graph).
The other distinct advantage to SWIR monitoring of hot processes is the ability to image at high speeds, i.e. recovery of chemicals. Solid-state InGaAs cameras, with their high-frame-rate capability, high resolution, low lag, long lifetime and superior signal-to-noise ratios, are now replacing commonly-used tube technology. InGaAs area arrays and linear arrays can reach tens of thousands of frames or lines per second, enabling high speed monitoring of fast-occurring phenomena. The newly introduced high speed, high resolution Windowing InGaAs camera from SUI (formerly Sensors Unlimited, Inc.), part of Goodrich Corporation (Fig. 2 –SWIR SU640SDWH High Speed, Windowing Camera) can image up to 15,000 frames per second with high linearity and excellent reliability. High-frame-rate, cryogenically-cooled InSb or HgCdTe cameras with windowing capabilities are 50% to 100% more expensive to purchase than comparable-sized InGaAs cameras. The cryogenically-cooled cameras are also much larger in size, require more power to operate and they have shorter overall lifetimes, due to their cooler assemblies. The uncooled microbolometer cameras do have lower overall costs than the cooled systems, but they are limited by their video field rate capabilities (e.g. 60 fields per second with a 320 x 256 pixel array).
Shortwave infrared imaging provides machine vision users with unique solutions for remote-sensing applications. Modern, solid-state SWIR cameras (see Figure 3 - InGaAs SWIR area and linescan cameras and arrays) based on InGaAs technology are small, light weight, and feature room temperature operation with low power consumption. SWIR-InGaAs real-time and/or high speed imaging for hot process monitoring results in low cost of ownership and helps ensure success for the end-user’s application.